Process for the improvement of the development of the texture of inductive surface-hardened steel parts

ABSTRACT

A METHOD AND APPARATUS FOR INDUCTIVELY SURFACE HARDENING A STEEL WORKPIECE WHEREIN THE WORKPIECE IS FIRST HEATED TO A TEMPERATURE ABOVE THE HIGHEST CRITICAL TEMPERATURE AC3, COOLED TO ATEMPERATURE BELOW THE LOWEST CRITICAL TEMPERATURE AC1, SO THAT THE STRUCTURE OF THE STEEL CONSISTS OF PEARLITE OR BAINITE, INDUCTIVELY REHEATED TO A TEMPERATURE ABOVE AC3 AND QUENCHED. PREFERABLY THE WORKPIECES ARE ELONGATED AND ARE CONTINUOUSLY ROTATED DURING HEATING AND COOLING. THE APPARATUS DISCLOSED INCLUDES A MACHINE WITH A NUMBER OF WORKING STATIONS FOR HEATING AND COOLING THE WORKPIECE AND A CARRIER SPIDER FOR MOVING THE WORKPIECES FROM STATION TO STATION.

Aug. 8, 1972 SEULEN ETAL 3,682,721

PROCESS FOR THE IMPROVEMENT OF THE DEVELOPMENT OF THE TEXTURE 0FINDUCTIVE SURFACE-HARDENED STEEL PARTS Filed Nov. 25, 1969 4Sheets-Sheet 1 Zia- 42 ya n $54 Aug. 8, 1972 SEULEN EIAL 3,682,721 F THEPROCESS FOR THE IMPROVEMENT OF THE DEVELOPMENT 0 TEXTURE 0F INDUCTIVESURFACE-HARDENED STEEL PARTS Filed Nov. 25, 1969 4 Sheets-Sheet 2 Fig!//7 van fans 678,96 62-145 Aug. 8, 1972 s u ETAL 3,682,721

PROCESS FOR THE IMPROVEMENT OF THE DEVELOPMENT OF THE TEXTURE 0FINDUCTIVE SURFACE-HARDENED STEEL PARTS Filed Nov. 25, 1969 4Sheets-Sheet 5 Aug. 8, 1972 SEULEN EIAL' 3,682,721

rnocmss FOR THE IMPROVEMENT OF THE DEVELOPMENT OF THE TEXTURE 0FINDUCTIVE SURFACE-HARDENED STEEL PARTS File'd Nov. 25, 1969 4Sheets-Sheet 4 //I v80 /0 rs Z-"ZAWEJ 5501:

United States Patent US. Cl. 148-131 6 Claims ABSTRACT OF THE DISCLOSUREA method and apparatus for inductively surface hardening a steelworkpiece wherein the workpiece is first heated to a temperature abovethe highest critical temperature AC cooled to a temperature below thelowest critical temperature AC so that the structure of the steelconsists of pearlite or bainite, inductively reheated to a temperatureabove AC and quenched. Preferably the workpieces are elongated and arecontinuously rotated during heating and cooling. The apparatus disclosedincludes a machine with a number of working stations for heating andcooling the workpiece and a carrier spider for moving the workpiecesfrom station to station.

The invention relates to an improved method of and apparatus forinductively surface hardening steel parts to obtain an improved grainstructure.

When steel parts are surface hardened by heating with currents inducedin the workpiece and then quenched, the grain structure of the metal inthe hardened zone may not be uniform if the initial structure prior tohardening was not uniform. Such non-uniformities may occur when thesteel parts which are to be surface hardened have been previously onlypartly raised to a proper forging temperature before forging and thencooled without sufficient control exercised It is also possible forparts that have been completely heated in forging to exhibit anonuniform initial grain structure if they have not been thereafteruniformly cooled, particularly if the cooling rate is not uniform. Thisformation of interphase structures and/or pearlite does not necessarilyhave significantly adverse consequences in a subsequent surfacehardening treatment. However, if a ferritic structure has formed to anyappreciable extent, then the relatively short time available foraustenite formation during inductive heating does not always assure auniform accumulation of austenite for the subsequent hardening. Theresult is that, when the part has a heterogeneous austenite phase with arelatively large proportion of bainite, the hardness achieved in theseregions is considerably less than the desired value thereby adverselyaffecting the wear resistance and/or fatigue strength of the treatedsteel part. These irregularities have a particularly undesirable elfectwhen the parts are likely to experience fatigue stressing. For instance,in long shafts such as side shafts, rear axle shafts, propeller shafts,as used in motor cars, torsional fatigue fracture regularly occurs inthose areas where the strength is impaired because of an insufiicientlydeveloped structure.

In order to eliminate these defects, the present invention provides anovel method of inductively surface hardening parts made of heat-treatedsteel wherein the development of the grain structure is improved. Thisis accomplished by first inductively heating the zone of the workpieceto be hardened by overall surface heating to a temperature above the ACtemperature, which is the highest critical temperature, and then coolingit to a temperature below the AC temperature, which is the lowestcritical temperature, in such a way that the strucure of the steel,after having cooled, has a pearlitic or a bainite. Subsequently the zoneis inductively reheated above the AC temperature and quenched-hardenedfrom this temperature in conventional manner, for example in water, oilor an emulsion.

A particular advantage of the method of this invention is that coolingfrom the first heating which begins at a temperature above the ACtemperature is accelerated because the cold core of the workpiece drawsthe heat from the heated zones, external cooling being simultaneouslycarried out, for example, in a current of air or a liquid mist in air.After the subsequent reheating of the zones that are to be hardened totemperatures above the AC point, the core will have a reduced strengthdue to its raised temperature. Final quenching first causes martensiteto form in the surface layers, and the reduced strength of the coreoffers a lower resistance to the compressive stresses in the transitionzones due to the formation of the martensite with the result that theresidual tensile stresses in these zones are less than would occur ifthe resistance of the core had not been reduced since the forcesrequired are distributed over wider areas of the core.

When the method of the invention is applied to elongate workpieces whichare already distorted prior to being thermally treated or which tend todistort during hardening, then a further feature of this inventionincludes straightening the workpiece during the repeated heating,cooling and quenching steps. During thermal treatment the workpieces mayrotate and a force acting radially to the workpiece axis can be appliedthereto by rolls to accomplish the straightening. Because of the raisedtemperature of the core and the resultant reduced strength of theworkpiece core, the straightening process is much more successful thanwhen the core is completely cold, and the generated straighteningstrains are less than in a cold straightened part.

The method of the. invention is not restricted to the use of anyparticular apparatus. However a further feature of the inventionprovides apparatus for carrying out the method, namely, an inductivehardening machine comprising at least five working stations and fittedwith a carrier spider having at least five arms fitted with workgripping spindles. For performing the heat treatment the machineincludes a loading and unloading station, two heating stations and twocooling stations.

The method of the invention may be alternatively carried out on twoseparate machines, each machine being provided with three workingstations and with a spider comprising at least three arms and fittedwith three pairs of work gripping spindles. The two machines may bepositioned in any convenient manner, e.g. side-by-side or in tandem. Themachine for performing the heat treatment includes a loading andunloading station, a heating station and two cooling stations and thehardening machine includes a loading station, a heating station and aquenching and unloading station. The invention is hereinafter furtherdescribed and illustrated in the accompanying drawings, of which:

FIG. 1 is a longitudinal section of part of a carrier spider for use inapparatus according to the invention;

FIG, 2 is a schematic side elevation of a five-station machine accordingto the invention;

FIG. 3 is a schematic side elevation of a carrier spider of a preheatingmachine for use according to the invention; and

FIG. 4 is a part sectional view of a hardening machine for use accordingto the invention.

Reference is now made to FIG. 1 wherein the manner in which theworkpiece 1 is mounted in a carrier spider which conveys it from onetreating station to the next is illustrated. In FIG. 1 only one pair ofarms 2, 2' of the spider is visible, but it will of course be understoodthat each spider has a pair of arms for each working station. Theworkpiece is mounted between the centers of a. pair of spindles 3, 3',so that, when force is applied to cause rod 4 to move in the directionindicated by arrow 5, the fixed rotary member 30 exerts a force of theopposite direction on rod 32, which then moves in a direction oppositeto rod 4, carrying with it spindle 3 so that the two spindles separateand the workpiece is released. A spring 34 urges spindle 3 in adirection opposite to arrow 5. The pair of arms 2, 2 of the carrierspider is represented in FIG. 1 in a position in which the workpiece isat a heating station and the workpiece 1 is shown located beneath aninductor 6 which is supplied with alternation current from a suitablesource.

FIG. 2 illustrates one embodiment of the invention. In this figure, itis assumed that the loading and unload ing station is station 27, atwhich point the workpieces to be treated are transferred to the spiderand the treated workpieces are removed. After a workpiece is loaded ontothe spider at station 27 the spiders rotate in the direction indicatedby arrow 29 to heating station 28 at which the first heat treatment isperformed. After being heated to a temperature above the AC temperatureby a conventional inductor 6 at station 28, the .workpiece is nextrotated to a cooling station 30 where it is cooled by a current of airor of a mixture of liquid and gas discharged by sprayer head 11. Afurther step rotation takes the workpiece to a second heating station 31where it is again inductively heated to a temperature above the ACtemperature, by an-inductor 6, this time for the purpose of beingsurface-hardened. After the workpiece has been thus heated, the nextindexing step of the spider lowers it into a quench bath, the level ofthe quench bath being approximately indicated at 32. The workpiece 1then moves to quenching station 33 which may contain a supplementarysprayer head 11. After quenching, the workpiece may be removed from thequench bath at the station 33 by means hereinafter described or theworkpiece may be rotated to and removed at loading station 27, after afurther indexing step of the spider. For the entire length of time theworkpiece is in the spider, it is preferably maintained in continuousrotation about its axis by any suitable means. Moreover, each arm of thecarrier spider is equipped with at least one pair of straightening rolls14 which apply to the workpiece during the heat treatment and thehardening process a mechanical straightening force acting radially tothe workpiece axis.

The invention may also be carried out in separate preheating andhardening machines. Details of one suitable preheating machine areillustrated in FIG. 3, in which spider workpieces that are to bepreheated are first loaded onto the Spider at station 7. The workpiecesare then rotated stepwise, moving first to a heating station 8 where theworkpiece is heated by inductor 6 above the temperature AC next indexingstep of the spider conveys the workpiece to a first cooling station 9where it is exposed to air or a mixture of liquid and gas from a sprayerhead 11. If the time provided by the indexing cycle at station 11 is notsufficient for cooling, the cooling process may be completed at anotherstation 10 where another sprayer head 11 is provided to cool theworkpiece. From station 10 the workpiece finally returns to station 7where it is unloaded and conveyed by any suitable means to a hardeningmachine such as shown in FIG. 4. For moving the workpieces to and fromthe spider, conventional conveyors such as conveyor chains and the like,may 'be employed. Preferably the workpieces are kept continuously inrotation about their axes, as indicated by the arrows, for the whole ofthe time they are mounted in the spider. Moreover, each arm of thespider is equipped with at least one pair of straightening rolls 14which during the heat treatment and also during the hardening of theworkpieces apply to the workpieces a mechanical straightening forceradial to the workpiece axis.

The inductive hardening apparatus shown in FIG. 4 contains as itsprincipal component a carrier spider 36 provided with three workholderdevices such as shown in FIG. 1. The workpiece, having been conveyed ona conveyor chain 15 to a transferring point 16, is mounted betweenspindles as in the FIG. 1 in the position marked W When the spider hasbeen rotated through an indexing step, which in the illustrated examplecorresponds to the workpiece reaches a position W in which, in FIG. 4,it is exactly below an inductor 6. The workpiece is heated to hardeningtemperature in the inductor 6 and subsequently the spider is againindexed through a similar angle so as to immerse the workpiece in aquench bath contained in a vessel 17. Having been quenched, theworkpiece in position W is released by the spider and transferred tofurther conveyor means which removes it fromvthe quench bath.

This further conveyor means in FIG. 4 consists of a cranked lever 18which is formed at one end with a fork 19 for engaging the workpiece.After having been quenched in position W the workpiece is released byits holders and the forked end 19 of the cranked lever 18 is designed soas to be located below the workholders when the release takes place. Thecranked lever 18 is articulatedly attached to the ends of the two swingarms 20 and 21 which are each attached at their other ends a torotatable shafts 22 and 23 respectively. The swing arm 20 isarticulatedly attached to the cranked lever at the vertex angle thereof,whereas the other swing arm 21 is articulated to the end of the crankedlever 18 remote from the fork 19. One of the two shafts, for instancethe shaft 22, is connected to drive means which are started as soon as aworkpiece has been deposited in the fork 19 of the cranked lever 18 andwhich rotate the shaft 22 and thereby deflect the swing arms 20 and 21through an angle of about 60 into the positions shown in dotted lines.The workpiece resting in the fork 12 is thus conveyed along asubstantially linear path 24 marked by a dot-dash line. ,That patharches towards its end causing the workpiece to slide out of the forkand to drop onto a slightly inclined chute 25. To prevent the workpiecefrom prematurely dropping out of the work due to a malfunction of theconveyor mechanism the chute 25 rearwardlyextends into the quenchingvessel. Since the crutch of the fork 19 moves very closely above theextended chute the workpiece cannot drop back into the quench.

After dropping the workpiece, the fork 19 returns to its illustratedreceiving position below the workholder and when it reaches thisposition a limit switch (not shown) preferably deactivates the conveyormeans. The

driving speed of the conveyor means is chosen so that the time requiredfor a to and fro motion of the fork 19 is less than the time thatelapses between indexing motions of the spider to ensure that the fork19 will always be in position at W below the workholder devices when thenext workpiece is released by the spider. For assisting the quenchingeffect and for agitating the quench, a sprayer head 26 is additionallyprovided at the quenching station.

Many changes and modifications of the above embodiments of the inventioncan be made without departing from the scope of the invention which isintended to be limited only by the scope of the appended claims.

What is claimed is:

1. A method of inductively surface hardening at least a portion of thesurface of a steel workpiece to obtain an improved grain structurecomprising the steps of:

inductively heating said overall surface portion of the workpiece to behardened to a temperature above the AC temperature so that the structureof the steel after cooling consists of pearlite or bainite structure,

cooling said surface portion to obtain said pearlite or bainitestructure,

inductively reheating said cooled portion to a temperature above the ACtemperature,

quenching the reheated portion to harden it and straightening saidworkpiece during said heating, cooling, reheating and quenching steps.

2. A method as in claim 1 including the step of continuously rotatingsaid workpiece during said steps of heating, cooling, reheating andquenching.

3. A method as in claim 1 wherein said workpiece is cooled, during saidcooling step, to a temperature below the AC temperature.

4. A method as in claim 1 wherein said cooling step includes the step ofapplying a fluid stream to said workpiece.

5. A method as in claim 1 wherein said step of straightening includesthe step of applying at least a single straightening roll to saidworkpiece so that said roll rides on the surface of said workpiece so asto impart thereto a force acting radially to the axis of said workpiece.

6. A method of inductively surface hardening overall at least anelongated portion of a steel workpiece to obtain an improved grainstructure comprising the steps of:

inductively heating the overall surface of said portion above the ACtemperature so that the structure of 5 the steel after cooling consistsof a pearlite or bainite structure, cooling said portion below the ACtemperature to obtain said structure,

inductively reheating said portion to a temperature above the ACtemperature, quenching the reheated portion to harden it,

continuously rotating said workpiece during said steps of heating,cooling, reheating and quenching, and

straightening said workpiece during said steps of heating, cooling,reheating and quenching.

References Cited UNITED STATES PATENTS 2,590,546 3/1952 Kincaid et a1148-144 3,169,893) 2/1965 Wuerfel 148l31 3,255,053 6/1966 Bard et a1.148-431 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

